Development of Co-Based Nanocrystalline Soft Magnetic Alloys for Extreme Environments for Aerospace Applications

Amorphous and nanocrystalline soft magnetic alloys are an area of interest due to their use in power electronics operating at high frequencies and temperatures while experiencing low losses. As extreme environment applications in aviation and space increasingly require high performance soft magnetic materials, new alloy development is necessary to ensure functional stability at high temperatures. These materials’ unique structure of a soft magnetic amorphous matrix with nanocrystallites randomly nucleated throughout, allows for enhanced exchange coupling throughout the matrix, increasing saturation magnetization and reducing losses. Co-based nanocrystalline alloys were chosen to develop as they show promise in high temperature applications with Curie temperatures approaching 1000°C. This work focuses on developing a soft magnetic material for an inductor application to be used in a Venus flagship mission. This component material is targeted to operate at 500°C for at least 60 days in the corrosive Venusian atmosphere.

Amorphous and nanocrystalline materials are manufactured via a melt spinning method, forming amorphous ribbon around 20µm thick. The ribbons go through post processing annealing treatments to induce primary crystallization which develops the nanocrystalline grains to induce intergranular exchange coupling, ensure microstructural stability, and increase ductility. At too high temperatures the ribbons experience secondary crystallization, forming hard non-magnetic intermetallic phases, which degrade the soft magnetic properties. This leaves a small processing window between primary and secondary crystallization temperatures to ensure magnetic and microstructural stability. At elevated temperatures in the presence of oxides and sulfides, the material reacts, forming corrosion products leading to material degradation. Chemical alterations are used to optimize magnetic, microstructural, and corrosive properties. Metalloid elements are glass formers, where their small size stabilizes the amorphous phase during casting. Early transition metal elements are added to increase high temperature microstructural stability and limit crystalline growth. They can also enhance corrosion resistance by forming protective oxide at the initial stage of oxidation. These groups of elements need to be balanced with late ferromagnetic transition elements to ensure both magnetic and microstructural stability.

In this presentation we overview the alloy development and optimization process using materials characterization to develop new stable compositions. These enhanced high temperature alloys are demonstrated and explained through a combination of chemistry optimization, thermodynamic simulation, and post processing annealing techniques such as strain and field annealing. Various elemental compositions were tested to push primary and secondary crystallization temperatures as high as possible. Through regression modeling and materials characterization techniques (TEM, DSC, XRD) specific early transition metals were shown to improve high-temperature stability at extended times. ThermoCalc software has been used to predict the glass forming ability, or the ability to form an amorphous phase during casting, of thousands of alloy compositions. Additional oxidation and corrosion tests were performed to understand the effects of the Venus environment using a high-tech chamber, Glenn Extreme Environments Rig (GEER). Advanced microstructure investigation methods such as SEM, TEM, and elemental analysis, such as EDS, and XPS were performed to investigate microstructure evolution. Co-based amorphous/nanocomposite alloys show significantly thinner oxide scales on their surfaces with promising high-temperature magnetic properties. Once a database of various alloys is developed machine learning techniques and adaptive learning paths can be used to fully optimize alloy compositions resulting in extreme environmental operation for extended times.